Bone marrow is a dose limiting normal tissue for both chemotherapy and wide field irradiation. These laboratory investigations study the response of hematopoietic stem cells to these agents. Current research is concerned with the renewal potential of the hematopoietic stem cell to determine whether stem cells are "immortal" or have a finite life span. Thus far, our data indicate that the latter is the case and that this life span can be remarkedly reduced by some aklylating agents. A second area of investigation has been concerned with the study of engraftment of isogenic bone marrow into part body irradiated mice. These studies reveal that such engraftment occurs primarily in an unirradiated site; the likelihood and extent of engraftment being proportional to the dose of radiation.